FIG. 1 is a first perspective view of a new and ornamental design for a foldable blade sharpener; FIG. 2 is a top plan view thereof; FIG. 3 is a bottom plan view thereof; FIG. 4 is a front elevation view thereof; FIG. 5 is a rear elevation view thereof; FIG. 6 is a first side elevation view thereof; and, FIG. 7 is a second side elevation view thereof.